The retirement of 20-term Democratic Rep. George Miller is likely to set off a battle royal for a rare open seat in Congress.

Miller's successor is expected to come from his party's ranks, given that half the registered voters in his Contra Costa County district are Democrats and just 25 percent are Republicans.

With California's new "top two" primary system, "the only question is whether it's going to be two Democrats up against each other in November or whether there will be so many of them that they will split the field and allow a Republican to slip in," said Nathan Gonzales, deputy editor of the Rothenberg Political Report, a nonpartisan journal that analyzes federal races.

The district is one of the more diverse in California, stretching from the gated communities of Blackhawk through middle-class Concord to the low-income neighborhoods of Richmond.

Among those whose names were mentioned Monday as possible candidates:

State Sen. Mark DeSaulnier, D-Concord: He has already declared that he is in the race, potentially forsaking a chance to be state Senate leader when the current president pro tem, Sen. Darrell Steinberg, D-Sacramento, is termed out this year.

"He would have many of the same constituencies as Miller, but he's not seen as progressive as Miller," said David McCuan, a professor of political science at Sonoma State University.

State Assemblywoman Joan Buchanan, D-Alamo: She will be termed out of her Assembly seat this year after running unsuccessfully for Congress in 2009 for Rep. Ellen Tauscher's former seat.

Money wouldn't be an issue. During the 2009 race, Buchanan loaned herself $850,000. She also got backing from Emily's List, a fundraising group that backs Democratic women who favor abortion rights.

"It's early, and she is still exploring what her options are," Michelle Henry, Buchanan's political adviser, said Monday.

State Assemblywoman Susan Bonilla, D-Concord: She has less name recognition than others, as her district stretches north into Solano County. Some analysts said her best play might be to run for DeSaulnier's state Senate seat should he replace Miller. Bonilla, who had planned to run for DeSaulnier's state Senate seat when he terms out in 2016, said Monday, "I will talk to my husband and family about the various options."

State Superintendent of Public Instruction Tom Torlakson: The former Antioch schoolteacher climbed the ranks of Contra Costa politics from the Board of Supervisors through the Legislature. A spokesman for the state Department of Education said Torlakson is "definitely not running" for Miller's seat.

Torlakson is up for re-election to the nonpartisan office this year. The Democrat is likely to have an easy time of it in a state where his party has a commanding lead in voter registration.

Supervisor John Gioia: He told The Chronicle on Monday that he is running for re-election to the Board of Supervisors and will not run for Congress.

Richmond Mayor Gayle McLaughlin: The highest-ranking Green Party member in the country is developing a national profile, thanks to Richmond's effort to invoke eminent domain for underwater mortgages. However, she said Monday she wasn't interested in Miller's seat.

Other names being mentioned include a pair of Republicans, Contra Costa District Attorney Mark Peterson - a former Concord councilman - and Supervisor Mary Piepho.

Piepho, however, said she wasn't interested. Peterson did not immediately respond to a request for comment.